We plan to continue our investigations into methods for improving the efficiency of cancer clinical trials and observational epidemiologic investigations, and to study the implications and properties of new methods under investigation. We shall be exploring both theoretical and practical approaches. In the former we shall examine techniques for optimizing the design of studies, in terms of the treatment allocation (in clinical trials) and the characteristics of the subjects selected (epidemiologic studies). In the latter we shall explore ideas for expanding the population base from which subjects may be drawn. In the clinical trials setting this will involve examining the appropriateness of protocol eligibility criteria. In the epidemiology setting we shall study methods for using prevalent cases for case-control studies. In all of these investigations we shall pay due attention to the practical implications of any new methods. We have access to important datasets from the Eastern Cooperative Oncology Group for both clinical trials and epidemiology applications.